1. Field of the Invention
The invention relates to a thin-film electromagnet and a switching device including the same, and more particularly to a switch for turning on or off a current signal covering a dc current to an ac current having a frequency in the range of zero to a GHz or greater, and a micro electronics mechanical system (MEMS) switch applicable to an optical device such as a semiconductor laser which is capable of varying a wavelength of laser beams, an optical filter and an optical switch.
2. Description of the Related Art
Many conventional MEMS switches include a thin-film electromagnet for turning on or off a switch by driving a movable portion by means of electrostatic force.
For instance, such a MEMS switch is suggested in U.S. Pat. Nos. 5,578,976, 6,069,540, 6,100,477, 5,638,946, 5,964,242, 6,046,659, 6,057,520, 6,123,985, 5,600,383 and 5,535,047.
A conventional MEMS switch such as that described in U.S. Pat. No. 5,578,976 will now be discussed. FIG. 18A is a plan view of a MEMS switch suggested in U.S. Pat. No. 5,578,976, and FIG. 18B is a cross-sectional view taken along the line 18B—18B in FIG. 18A.
The MEMS switch illustrated in FIGS. 18A and 18B includes a substrate 101, a support 103 formed on the substrate 101, and a cantilever arm 104 swingable about the support 103.
On the substrate 101 are formed a lower electrode 102 composed of gold and signal lines 106 composed of gold.
The cantilever arm 104 comprised of a silicon oxide film is fixed at its fixed end to the support 103, and has a free end facing the signal lines 106. That is, the cantilever arm 104 extends to a point located above the signal lines 106 beyond the lower electrode 102 from the support 103, and faces the lower electrode 102 and the signal lines 106 with a spatial gap therebetween.
On an upper surface of the cantilever 104 extends an upper electrode 105 composed of aluminum from the support 103 to a location facing the lower electrode 102. On a lower surface of the cantilever 104 is formed a contact electrode 107 composed of gold such that the contact electrode 107 faces the signal lines 106.
The MEMS switch having such a structure as mentioned above operates as follows.
Applying a voltage across the upper electrode 105 and the lower electrode 102, attractive force caused by electrostatic force acts on the upper electrode 105 towards the substrate 101 (in a direction indicated with an arrow 108). As a result, the cantilever 104 deforms at its free end towards the substrate 101, and thus, the contact electrode 107 makes contact with facing ends of the signal lines 106.
In non-operation condition, since the gap separates the contact electrode 107 and the signal lines 106 from each other, the signal lines 106 are electrically insulated from each other. Accordingly, when a voltage is not applied across the upper electrode 105 and the lower electrode 102, a current does not run through the signal lines 106.
When a voltage is applied across the upper electrode 105 and the lower electrode 102 to thereby cause the contact electrode 107 to make contact with the signal lines 106, the signal lines 106 are electrically connected to each other through the contact electrode 107, resulting in that a current runs through the signal lines 106.
As explained above, it is possible to control the on/off status of a current or signal running through the signal lines 106, by applying a voltage across the upper electrode 105 and the lower electrode 102.
However, the conventional MEMS switch making use of electrostatic force, illustrated in FIGS. 18A and 18B is accompanied with the following problems.
First, the attractive force is small, because it is derived from electrostatic force.
FIG. 21 is a graph showing the dependency of electrostatic force and electromagnetic force on a size.
As is obvious in view of FIG. 21, electrostatic force is smaller than electromagnetic force by one to three column(s) in a size in the range of tens of micrometers to hundreds of micrometers to which a MEMS switch is applied.
A relay switch to which the MEMS switch illustrated in FIGS. 18A and 18B is applied is said to be required to have a contact pressure of about 10−2 N in order to suppress contact resistance in an electrical contact and accomplish adequate electrical connection.
It is understood in view of FIG. 21 that if a distance between electrodes is 100 micrometers and a contact area is 10,000 square micrometers, there is obtained a force of about 10−5 N, even if a voltage of 3×106 V/cm is applied across the electrodes.
Second, a high voltage is maintained across the lower electrode 102 and the upper electrode 105 in order to keep the MEMS switch illustrated in FIGS. 18A and 18B on.
This means that electric power is always consumed. In addition, application of a high voltage across electrodes facing each other with a small gap therebetween creates problems such as destruction of a device caused by generation of surge current.
Third, even if a high contact pressure is not required unlike a relay switch, a digital micro-miller device (DMD) suggested, for instance, in U.S. Pat. Nos. 5,018,256, 5,083,857, 5,099,353 and 5,216,537 is accompanied with a problem that a pair of electrodes are absorbed to each other when they make contact with each other by electrostatic force, and thus, they cannot be separated from each other by electrostatic force with the result of inappropriate operation.
A solution to the problem unique to DMD is suggested, for instance, in U.S. Pat. Nos. 5,331,454, 5,535,047, 5,617,242, 5,717,513, 5,939,785, 5,768,007 and 5,771,116.
A digital micro-miller device (DMD) is a smallest device among MEMS devices, and has a movable portion having a size of a few micrometers. Hence, a digital micro-miller device can obtain relatively high electrostatic force. Accordingly, it is not always possible to apply the solution unique to a digital micro-miller device to a MEMS switch having a size of about 100 micrometers or greater.
Fourth, a device which operates in analogue manner, such as an optical switch including a MEMS mirror suggested in U.S. Pat. No. 6,201,629 or 6,123,985 can have just a limited controllably operational range.
Supposing two electrodes arranged to face in parallel with each other, if a distance between the two electrodes becomes smaller than two thirds of an initial distance, the two electrodes rapidly make contact with each other, resulting in inability of control in operation of the electrodes. This is a general principle which can be analytically obtained.
Hence, if a swingable angle of a MEMS mirror is made greater, a distance between the electrodes has to be made greater, resulting in that a device including the MEMS mirror has to operate in a range in which electrostatic force is small. In contrast, if a device is designed to include a MEMS switch having a small swingable angle, an optical switch which is often required to be arrayed in a large scale such as 1000×1000 or 4000×4000 has to have a large-sized switch. This is not practical.
As explained above, there are caused a lot of critical problems due to electrostatic force in a size of a MEMS switch in the range of a few micrometers to hundreds of micrometers.
One solution to these problems is to use electromagnetic force in place of electrostatic force.
As shown in FIG. 21, electromagnetic force is greater than electrostatic force by one to three column(s) in a size in the range of tens of micrometers to hundreds of micrometers to which a MEMS switch is applied. U.S. Pat. No. 6,124,650 describes a MEMS switch in which electromagnetic force is used. Such a MEMS switch is illustrated in FIG. 19.
On a substrate 201 are formed a plurality of current wires 203, and a cantilever arm 202 bridging over the current wires 203. A magnetic layer 204 is formed on the cantilever arm 202, and an electrical contact 206 is formed on the cantilever arm 202 at a distal end thereof. On another substrate 208 fixed relative to the substrate 201 are formed a magnetic layer 205 facing the magnetic layer 204, and an electrical contact 207 facing the electrical contact 206. The magnetic layer 204 is composed of soft magnetic substance, and the magnetic layer 205 is composed of hard magnetic substance.
The MEMS switch illustrated in FIG. 19 operates as follows.
The magnetic layer 204 is magnetized in a direction due to a magnetic field generated by a current running through the current wires 203. For instance, the magnetic layer 204 is magnetized to have N-polarity at its left end in FIG. 19, and S-polarity at its right end in FIG. 19.
Contrary to the magnetization of the magnetic layer 204, the magnetic layer 205 is magnetized in advance to have S-polarity at its left side and N-polarity at its right side. Thus, attractive force is generated between the right end of the magnetic layer 204 and the right end of the magnetic layer 205, and hence, the cantilever 202 is bent towards the substrate 208 located thereabove. As a result, the electrical contacts 206 and 207 make contact with each other to thereby turn a switch on. Even if a current running through the current wires 203 is shut off, since the magnetic layers 204 and 205 have remanent magnetism, the switch is kept on.
By making a current run through the current wires 203 in the opposite direction, remanent magnetism in the magnetic layer 204 is reduced as the current is gradually increased, and then, a force making the cantilever arm 202 return to its original position exceeds the attractive force generated between the magnetic layers 204 and 205. If the current running through the current wires 203 is shut off in such a condition, the electrical contacts 206 and 207 are separated from each other, and thus, the switch is turned off.
However, the MEMS switch illustrated in FIG. 19 has the following associated drawbacks.
First, when the magnetic layer 204 is magnetized by a magnetic field generated by the current running through the current wires 203, it would not be possible to sufficiently magnetize the magnetic layer 204, because the magnetic layer 204 has an intensive diamagnetic field.
This is because of dimensional limit caused by the arrangement in which the magnetic layer 204 is formed on the cantilever arm 202.
In order to weaken a diamagnetic field for sufficiently magnetizing the magnetic layer 204 by a magnetic field generated by a weak current, the magnetic layer 204 has to be formed lengthy in a direction of magnetization and thin.
However, if the magnetic layer 204 is so formed, magnetic flux which the magnetic layer 204 originally generates is reduced. As a result, the attractive force between the magnetic layers 204 and 205 is reduced.
In contrast, if the magnetic layer 204 is formed wider and thicker, a diamagnetic field would be greater, and hence, it would be necessary to make a current run through the current wires in a larger amount in order to magnetize the magnetic layer 204, resulting in an increase in power consumption.
As explained above, the MEMS switch illustrated in FIG. 19 is accompanied with the antinomic problem.
Second, the MEMS switch illustrated in FIG. 19 is difficult to fabricate.
This is because the cantilever arm 202 acting as a movable portion is designed to be arranged in a space formed between the fixed substrates 201 and 208.
As illustrated in FIG. 19, in the process of fabrication of the cantilever arm 202, there is first formed a sacrificial layer which will be removed in a final step of the process, and then, the cantilever arm 202, the magnetic layer 204 and the electric contact 206 are formed on the sacrificial layer. Then, another sacrificial layer is formed on the cantilever arm 202, and then, the substrate 208 including the magnetic layer 205 and the electrical contact 207 is formed on the sacrificial layer. In a final step of the fabrication process, the two sacrificial layers formed on and below the cantilever arm 202 are removed by etching, for instance.
When the sacrificial layers are removed, there are caused two problems as follows.
The first problem is that surfaces of the cantilever arm 202 and the substrates 201 and 208 are contaminated, and etching residue and re-formed deposit are adhered to the surfaces, after the etching has been carried out. As a result, there are caused many troubles such as degradation of the electrical contacts 206 and 207, defective operation of the cantilever arm 202 as a movable portion, and adsorption of adhesive contaminants to the cantilever arm 202.
The second problem is that when the sacrificial layers are wet-etched or when the sacrificial layers are wet-washed after dry-etched, the cantilever arm 202 is adsorbed to the substrate 201 or 208 because of surface tension of an etchant or a washing solution, and thus, cannot be peeled off the substrate 201 or 208.
The above-mentioned two problems are caused by the arrangement that the cantilever arm 202 acting as a movable portion is located between the fixed substrates 201 and 208, and are frequently caused with the result of reduction in a fabrication yield and increase in fabrication costs.
As a solution to the above-mentioned problems, there is a process in which the substrate 208 including the magnetic layer 205 and the electrical contact 207 is fabricated separately from the substrate 201 including the cantilever arm 202 and the current wires 203, and the substrates are adhered to each other in a final step.
However, the process requires a doubled number of ceramic wafers which will make the substrates 201 and 208, resulting in an unavoidable increase in fabrication costs.
In addition, the arrangement of the cantilever arm 202 between the fixed substrates 201 and 208 makes it difficult to observe and inspect the cantilever arm 202. Hence, it would be difficult to check defects such as the above-mentioned adsorption, preventing analysis of a cause of the defects. This results in further reduction in a fabrication yield and further increase in fabrication costs.
U.S. Pat. No. 6,124,650 suggests such a MEMS switch as illustrated in FIG. 20.
In the MEMS switch, a plurality of current wires 303 is formed on a substrate 301, and a cantilever arm 302 bridges over the current wires. A magnetic layer 304 is formed on an upper surface of the cantilever arm 302, and an electrical contact 307 is formed on a lower surface of the cantilever arm 302 at a distal end.
A magnetic layer 305 is formed on the substrate 301, facing a part of the magnetic layer 304, and an electrical contact 306 is arranged in facing relation to the electrical contact 307. The magnetic layer 304 is composed of soft magnetic substance, and the magnetic layer 305 is composed of hard magnetic substance.
The MEMS switch illustrated in FIG. 20 solves the above-mentioned second problem, but cannot solve the above-mentioned first problem.
In view of the above-mentioned problems in conventional switching devices, it is an object of the present invention to provide a MEMS switch which is capable of accomplishing wide-range movement by virtue of attractive and repulsive forces, is suitable to an optical switch, a relay switch, a semiconductor laser irradiating laser beams having a variable wavelength, and an optical filter, and can be readily fabricated.